The invention resides in an internal combustion engine with exhaust gas turbocharger and compound power turbine, which is coupled to the crankshaft of the engine and is disposed downstream of the exhaust gas turbocharger so as to receive the exhaust gas therefrom to be driven thereby.
Such an internal combustion engine is known from DE 195 16 971 A1. It discloses an internal combustion engine with an exhaust gas turbine arranged in the exhaust duct and connected to a compressor, which is disposed in the intake duct of the engine. Downstream of the exhaust gas turbine in the exhaust gas duct is a compound power turbine, which is coupled to the crankshaft of the engine by a transmission. The compound turbine permits the utilization of the residual energy contained in the exhaust gas after passage through the exhaust gas turbine and to transfer this energy to the crankshaft of the engine as a positive drive moment or as a negative brake moment. The total efficiency of the engine is improved by such a series arrangement of exhaust gas turbine and compound power turbine.
With the internal combustion engine as disclosed in DE 195 16 971 A1, the engine drive and engine braking power can be increased. However, a reduction of the NOx emissions, which can be achieved by exhaust gas recirculation, is not provided for and cannot be easily realized without detrimentally affecting the engine efficiency since the pressure loss caused by the exhaust gas recirculation in the exhaust pipe ahead of the exhaust gas turbine prevents the generation of any significant power in the compound power turbine.
DE 195 16 911 A1 discloses no possibility for solving the conflict between reduced emissions on one hand and good engine efficiency on the other.
It is the object of the present invention to provide an internal combustion engine with an exhaust gas turbocharger and a compound power turbine, which provides for a low level of noxious emissions and a good overall efficiency during engine power operation and also during engine braking operation.
In an internal combustion engine including an exhaust gas turbocharger and a compound power turbine connected to the crankshaft of the engine and an exhaust gas recirculation system extending between the engine exhaust duct upstream of the exhaust gas turbine and the intake duct downstream of the compressor of the exhaust gas turbocharger, the exhaust gas turbine and the compound power turbine have a size relationship determined by the power absorption capability xcfx86 of each turbine such that:   φ  =            m      .        ⁢                  T            p      
wherein: xe2x80x94{dot over (m)}xe2x80x94 is the exhaust gas mass flow rate through the turbine,
T is the exhaust gas temperature at the turbine inlet, and
p is the exhaust gas pressure at the turbine outlet and the maximal gas turbine exceeds the maximal absorption capability of the exhaust gas turbine.
The exhaust gas turbine and the compound power turbine are provided in a certain size relationship which is determined mainly by the maximum power absorption capacity of each turbine from a known relationship depending on the exhaust gas mass flow through the turbine, a square root function of the temperature at the turbine inlet and a reciprocal value of the pressure at the turbine inlet. The maximum absorption capability is determined for the exhaust gas turbine and also for the compound power turbine, wherein the size ratio of exhaust gas turbine and compound power turbine is so established that the maximum power absorption capability of the compound power turbine exceeds the maximum power absorption capability of the exhaust gas turbine. If the turbines are sized in accordance with this dimensioning rule a high exhaust gas back pressure between the cylinder outlet of the internal combustion engine and the exhaust gas turbine can be achieved as it is required for exhaust gas recirculation and, consequently, for a reduction of nitrogen oxides in the exhaust gas. If this dimensioning rules is observed, the exhaust gas turbine is further small enough to cause a sufficient exhaust gas backup and a corresponding exhaust gas back pressure as it is needed for the exhaust gas recirculation.
On the other hand, the exhaust gas turbine may be sized large enough to generate a high charge pressure in the air intake duct of the internal combustion engine as it is needed for a high engine power output as well as for a good engine braking performance.
In a preferred embodiment, the absorption capability of the exhaust gas turbine is about 45% to 55% of the maximum power absorption capability of the compound power turbine. This size ratio is considered to be an optimum solution of the conflicting aims to provide, on one hand, a small exhaust gas turbocharger which can generate a high back pressure and, on the other hand, a large exhaust gas turbocharger which can generate a large intake air pressure. With the additional serially arranged compound power turbine the overall engine efficiency can be improved since the compound power turbine generates additional drive power and additional drive torque when the engine is operating and also, during braking operation, a braking torque which is applied directly to the crankshaft.
In a preferred embodiment, the exhaust gas turbine is provided with a variable vane structure for adjusting the effective turbine inlet flow cross-section. The variable vane structure (turbine geometry) permits to control the power setting during engine power operation and also during engine braking operation. In connection with the exhaust gas recirculation, an exhaust gas back pressure which exceeds the charge air pressure and consequently exhaust gas recirculation can be achieved over a large operating range by an adjustment of the turbine geometry to a back-up position which optimizes the turbine inlet flow cross-section. A deterioration of the charge change is compensated for by the power generated by the serially arranged compound power turbine so that, in addition to the reduction of the NOx emissions, the overall efficiency can be maintained with exhaust gas recirculation or even improved.
The compound power turbine is preferably so designed that, in the essential engine performance graph areas, the engine can be operated close to the optimal efficiency. To this end, the fast speed number of the compound power turbine defined as quotient of circumferential speed at the turbine wheel inlet and isentropic speed, is, at full engine load, in the area of 0.55 to 0.60. The isentropic speed is that theoretical speed, which is obtained if the turbine pressure drop is converted into flow speed without losses. With such a dimensioning, the compound power turbine operates, upon an engine torque reduction, also without changes of the variable geometry in the area of optimal efficiency with a fast speed number of about 0.7. With a further reduction of the engine torque, the fast speed of the compound power turbine increases at constant torque engine speed above the optimal value. With an adjustment of the variable turbine geometry toward opening however a decrease of the efficiency of the compound power turbine can be counteracted by an increase of the pressure drop by means of the compound power turbine.
With the use of a variable turbine geometry in the exhaust gas turbine, the maximum absorbability of the exhaust gas turbine can be provided for a maximally opened variable turbine geometry as well as for a back-up position with minimum flow cross-section. With maximally opened turbine geometry, the maximal absorbability is preferably 45% to 55% of the maximal absorbability of the compound power turbine. For the back-up position, it has been found to provide a maximal absorbability of the exhaust gas turbine of at least 10% of the maximal absorbability of the compound power turbine in order to achieve, particularly under engine braking operation, a significant engine braking effect at lower engine speeds.
Further advantages and embodiments of the invention will become apparent from the following description of a particular embodiment thereof on the basis of the accompanying drawings.